I was hoping that you might get official responses to your questions (3) and (4) above. (Even though official replies are not the norm in the community forum, of course.)
In the interim…
This unfortunately isn’t conclusive, since a redirect is counted as a failed health check. (Which surprises a lot of people.)
The best way to verify is from inside the machine itself…
$ fly ssh console
# apt-get update
# apt-get install --no-install-recommends curl
# curl -i 'http://localhost:3000/up' # no `-L` (!)
(Assuming a Debian-based image.)
The response needs to be a straight 200 OK; even a 301 or 302 redirect will count as unhealthy, 
.
Hope this helps a little!